Until now, in order to carry and use a small electric device, various primary and secondary batteries have been used. However, the power consumption of recent small electric devices has increased due to their high performances, and therefore, it has become impossible for small and light-weight primary batteries to supply a sufficient amount of energy. On the other hand, although there is an advantage in using a secondary battery in that it can be recharged, usable energy upon one recharge is even less than that of a primary battery. Furthermore, not only another power source is necessary for charging a secondary battery, but also charging usually takes several tens of minutes to several hours, and it is difficult to make it possible to always use it immediately anywhere.
Furthermore, the tendency to carry and use the electric devices has increased with the advent of a wireless network environment, requiring even a further reduction in the size and weight of these devices. Hence, it is difficult for conventional primary and secondary batteries to supply sufficient energy for driving such devices.
As a solution to this problem, small fuel cells have attracted attention. Until now, fuel cells have been developed as driving sources of large-sized dynamos and automobiles. This is mainly because fuel cells have high generating efficiencies in comparison with conventional power generation systems, and moreover, their byproducts are clean. On the other hand, a reason why a fuel cell system may be useful as a driving source of a small electric device is that an energy amount per volume or per weight that can be supplied by the fuel cell is several times to tens of times that of a conventional battery. Furthermore, since it is possible to continuously use a fuel cell so long as the fuel is replaced, there is no charging period as with secondary cells.
Although various types of fuel cells have been invented, for example, polymer electrolyte fuel cells are suitable for a small electric device, and in particular, a portable device which is carried and used. This is because these fuel cells can be used at near room temperature, and in addition, since their electrolyte is not a liquid but a solid, they have an advantage in that they can be safely carried.
Methanol has been studied as a fuel for the fuel cell system for a small electric device. This is mainly because methanol is a fuel that can be easily stocked and acquired.
It is best to use hydrogen as fuel in the fuel cells for obtaining a large output. However, hydrogen is a gas at room temperature and it was very difficult to store hydrogen in a small fuel tank at a high density.
One conventional hydrogen storage method involves compressing and saving hydrogen as a high pressure gas. However, the volume hydrogen density is about 18 mg/cm3 even if gas pressure is increased to 20 MPa (about 200 atmospheres).
A second method of storing hydrogen involves keeping it at a low temperature as a liquid.
A third method is a method of storing hydrogen by using a hydrogen storing metal alloy. According to this method, the occlusion amount per volume is large.
A fourth method involves loading methanol, gasoline, or the like, in a fuel tank and converting it into hydrogen for use.
A fifth method is a method of using a carbon-based material, such as a carbon nanotube, a graphite nanofiber, or a carbon nanohorn. These carbon-based materials can occlude hydrogen at about 10% by weight. Accordingly, when the fuel cell is used as a power source in a digital camera, it is possible to take about 3 to 5 times more photographs than in the case of using a conventional lithium ion battery.
In addition, a sixth method is a method of using a chemical hydride. The chemical hydride is a compound that occludes and releases hydrogen by using a chemical reaction. There are various organic materials and inorganic materials that can be broadly classified as chemical hydrides. An example of an inorganic chemical hydride is a boro hydride. Organic chemical hydrides may be, for example, cyclohexane, decalin and the like. These compounds can occlude about 5 to 10% by weight of hydrogen.
In addition, although a cell unit of a fuel cell system comprises at least one fuel cell, an amount of power generation of about 5V is usually needed so as to drive a mobile device. Since the amount of power that can be generated by one cell is about 1V at the maximum, it is necessary to connect a plurality of cells in series in order to obtain the needed amount of voltage.
The above-described fuel cell system comprises respective units, such as a cell unit comprising one or more fuel cells, a fuel tank unit for storing a fuel, a fuel feed unit for supplying the fuel of the fuel tank unit to the cell unit, an opening for supplying an oxidizer gas to the cell unit, and a wiring unit for collecting a generated power. However, in such a structure of the above-described respective units of the fuel cell system, the shape of a fuel cell system for mounting in a small electric device, the structure of arrangement of respective units in the battery, and in particular, the structure of arrangement of respective units necessary for miniaturization were not taken into consideration.
In order to efficiently arrange a plurality of fuel cells, until now, a method of stacking, in turn, an MEA (Membrane Electrode Assembly) comprising an electrode and a polymer electrolyte membrane, and a separator in which a fuel flow path is located on the side of the partition wall of the fuel, has been adopted. In addition, the cells stacked were electrically connected in series by producing the separators with conductive materials.
This embodiment is shown in FIG. 12. FIG. 12 is a schematic sectional view showing the structure of respective fuel cells stacked in a conventional fuel cell system. As shown in this figure, a cell unit 1 of a fuel cell system comprises one or more fuel cells 14, and one fuel cell 14 has an oxidizer electrode 11 in one face, a fuel electrode 13 in another face, and is provided with an oxidizer flow path 44 for taking in air on the side of the oxidizer electrode 11 and a fuel flow path 43 for supplying fuel on the side of the fuel electrode 13. The fuel cell system is formed by stacking the above-described one fuel cell as a constitutional unit with interposition of a separator 45 between the fuel cells. Thus, respective separators 45, which separate fuel cells, are provided between each adjacent fuel flow path 43 and oxidizer flow path 44 of the stacked fuel cells. In addition, for each fuel cell 14, the oxidizer flow path 44 and fuel flow path 43 are independently provided, respectively, and three separators 45 are provided for four sheets of fuel cells 14. Therefore, since the thickness of the entire fuel cell system is the sum of the thicknesses of the fuel cells, oxidizer flow paths, fuel flow paths, and separators, there is a problem in that the size of the fuel cell system becomes large with respect to its capacity to generate electric power.
Moreover, regarding the size of a fuel cell system, as a miniaturization method of the fuel cell system, in “Planar Interconnection of Multiple Polymer Electrolyte Membrane Fuel Cells by Microfabrication” presented by F. B. Prinz et al. in the 2001 Joint International Meeting, “the 200th meeting of The Electrochemical Society Inc. and the 52nd Annual Meeting of the International Society of Electrochemistry” held in September, 2001, a method of locating a plurality of fuel cells on the same plane was also attempted. In this case, so as to collect the electricity generated in the cells, a method of using three-dimensional wiring, etc., and performing serial wiring was adopted.
However, in such a configuration, the structure for mounting in a small electric device and the structure necessary for miniaturization were not taken into consideration. In particular, a stacking method of fuel cells in a conventional fuel cell system had a defect in that fuel could not be efficiently supplied to the cells since the fuel flow paths became narrow when the fuel cells had a small volume and the number of stacks increased.
Moreover, the method of placing a plurality of fuel cells on the same plane, and connecting them in series using three-dimensional wiring, etc., required holes in the cells for sending the electric current, destroying the sealing performance of fuel chambers.